A tachometer for indicating the speed in revolutions per minute of an internal combustion engine normally consists of an instrument which is mechanically connected to and driven by the crankshaft or other rotating element in the engine. Another known type of tachometer consists of a magnetic pick-up positioned to generate an electrical pulse every time a notch on the engine vibration damper passes the pick-up. The electrical pulses per unit of time are easily translated to revolutions per minute. Yet enother tachometer picks up ignition pulses from the ignition system and translates the pulses to engine speed. These known tachometers all require a mechanical or electrical connection to an often inaccessible point on the engine, or require coupling to an ignition system which does not exist in compression-ignition diesel engines.
There is a need for a tachometer which can indicate engine speed without requiring the making of any mechanical or electrical-physical connection to the engine. Such a tachometer is particularly needed in conjunction with the full-load, full-speed testing of the power of internal combustion engines for diagnostic and maintenance purposes.
The full-load testing of internal combustion engines at maximum safe speed can be accomplished by connecting the engine to a dynamometer capable of absorbing the full-load output of the engine and capable of measuring the speed, torque and horsepower of the engine. Dynamometers are very large, very expensive, and very inconvenient to use.
A much more economical and convenient apparatus for testing spark-ignition internal combustion engines under simulated full-load conditions is apparatus for operating the engine with all but one of n ignitions interrupted, where n may be greater than the number of cylinders, so that all cylinders are operated in sequence under full power conditions. The engine operates at full speed driving frictional and pumping loads, without danger of overspeeding and damaging the engine.
An economical and convenient test of compression-ignition internal combustion engines under simulated full-load conditions is the acceleration burst test in which an engine initially operating at idle speed is suddenly given full throttle and caused to accelerate to a maximum governed speed. The inertia of the engine is the load on the engine, and the time taken to accelerate through a low speed to a high speed is a measure of the full-power, full-load condition of the engine. This test is particularly useful for testing diesel engines, and provides a somewhat less accurate indication of the condition of a spark ignition engine equipped with a carburetor instead of fuel injectors.
The acceleration burst test itself can be completed in about one second, once the speed-sensing tachometer and the computer are installed and connected. It is clear that the existing known methods of physically attaching a tachometer to an engine are relatively very time consuming and troublesome. It is therefore highly desirable to have a tachometer which is almost instantly operative without requiring any mechanical or electrical connection to the internal combustion engine.